O2 sensors do wear out and eventually need to be replaced. The performance of the O2 sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce it's voltage. This is caused by substances such as lead, silicone, sulfur, oil ash and even some fuel additives. The sensor may also be damaged by water, road salt, oil and dirt.
The best way to validate a O2 sensor is functioning normally is to check responsiveness to changes in the air/fuel mixture with an oscilloscope. if an oscilloscope is not available, the O2 sensor's output voltage can be read with a scan tool or digital voltmeter, but the transitions are hard to see because the numbers fluctuate quite a bit. And it is another method using LM3914N dot/bar display driver.
Step 1: Parts
To build this O2 sensor tester you will need :
1) 1x lm3914N - dot/bar display driver
2) 1x 78L05 - voltage regulator
3) 1x DL4148 - diode
4) 2x 10k variable resistor
5) 3mm led ( green, yellow red)
6) copper board or perforated breadboard
Step 2: PCB Board and Schematic Diagram
Design printed circuit board (PCB), I was used ,,Sprint-Layout" software.
Step 3: Final Testing
A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum (0.1 v) to near maximum (0.9v). Making the fuel mixture artificially rich by feeding propane into the intake manifold should cause the sensor to respond almost immediately (1/10 sec) and go to maximum (0.9v) output. Creating a lean mixture by opening a vacuum line should cause the sensor output to drop to its minimum (0.1v) value. A sensor that does not react quickly should be replaced.